He has to tell someone else.

First, do you have an eye witness?  Video is not the only useful evidence. 

Second, get a party admission, i.e. admission by the alleged party.  Wear a wire, or have someone else wear a wire, and ask the suspect in order to record his/her admission.  See http://en.wikipedia.org/wiki/Party_admission

Third, introduce statement against interest, see distinction in wikipedia article. Another witness can testify, that the alleged told the witness about the bad act.  This is only admissible when the alleged party is unavailable (e.g. refuse to testify).  

All of the above are admissible in court.